I managed to accidentally use this path without even trying, and it netting me around 100K per five minutes.

Here's what I did.

1.) Start up Ultimate Battle Tournament.
2.) Defeat Marduk, which will net you around 100K. (note that this is how much I said before)
3.) Go to the fight with battle ax vladitor.
4.) Lose the fight.
5.) click return to go back to a main menu.
6.) Reenter the UBT, and you'll have to fight Marduk again.
7.) Now you'll win 100K each time for beating him.
8.) Repeat as much as needed. This way helped me get my collect achievements before my achievements for 50,20,20 battles won.

A little tip that I used for all the tournaments:

Try not to do so many fights. Try to get double stands. Do this by knocking your opponent's bakugan into a place where it can't land. This is how I won against Marduk effectively.